Disk drives typically use heads residing on sliders to read from and write to the magnetic media. Read and write transducers residing in the head are flown at a small, controlled spacing above the magnetic medium during read and write operations. An air bearing forms between the head and the disk due to the disk rotating at high speeds. It may be desirable to estimate a fly height of the head, for example, when a system controls the fly height in order to optimize the write/read signals. To determine the controlled spacing, it may be desirable for the disk drive to detect at what point the head contacts the media, known as a touchdown event. Touchdown sensing may be accomplished by detecting actual contact or by proximity sensing methods. In addition, touchdown may be determined without dedicated sensors, for example using readback signal from the read transducer, position error signal (PES) from the servo, or other means.
For example, heads may use a thermal actuator that generates heat to control the head-media spacing. Heat generated by the thermal actuator causes local thermal expansion of the head, which locally reduces the spacing between the head and magnetic media. The thermal actuator can be driven to induce sufficient heating for contact or touchdown between the head and media. This touchdown is intentional and may be performed on each drive during initial drive calibration, or periodically during normal operation.
Thermal actuators cause touchdown and sensors detect touchdown; however, this does not ensure that the point where touchdown is made is actually sensed at the same location, or sensed at the desired location, or is well correlated to the read/write transducers fly height. In one example, touchdown may occur while the read sensor is still well above the surface, resulting in sub-optimal read signal. In another example, touchdown may occur at the read sensor first and cause damage to the read sensor.